An automotive battery is a rechargeable battery that supplies electric energy to an automobile. Traditionally, this is known as SLI, for starting, lighting, ignition, and its main purpose is to start the engine. Once the engine is running, power for the car is supplied by the alternator. Typically, starting discharges less than three per cent of the battery capacity. SLI batteries are designed to release a high burst of current, measured in amperes, and then be quickly recharged. They are not designed for deep discharge, and a full discharge can reduce the battery's lifespan.
As well as starting the engine an SLI battery supplies the extra power necessary when the vehicle's electrical requirements exceeds the supply from the charging system. It is also a stabilizer, evening out potentially-damaging voltage spikes. While the engine is running, most of the power is provided by the alternator, which includes a voltage regulator to keep the output between 13.5 and 14.5 V.
Howbeit, typical passenger vehicle and light truck alternators use Lundell or 'claw-pole' field construction. Modern vehicles now use the compact alternator layout, whereas larger vehicles may have salient pole alternators similar to larger machines.